Abstract: A model for defining environmental flow thresholds is developed using the abrupt habitat change analysis approach.The model takes into account the characteristics of aquatic ecosystems during spring flood periods,and uses the instream flow incremental methodology combined with the River2D model for fish habitat and the Mann-Kendall Method to establish the dynamic relation between the habitat and runoff.The model is tested on the data from the Wudaogou station located at the downstream of Huifa River.The data includes the streamflow velocities and water levels representing the habitat of the carp fish,as well as the monthly runoff of spring flood period from 1956 to 2000. The result of the Mann-Kendall analysis shows that the habitat of carp has undergone an abrupt change beginning in 1970.Prior to the abrupt change,the 95% confidence interval on the weighted usable area ranges from 275.5 to 915.7 m²/km,and the corresponding streamflow discharges taken as the environmental flow thresholds are from 58.8 to 121.1 m³/s.Spring flooding occurs after the icebound season,and the appropriate environmental flow of spring flood period is very important for the aquatic organisms and the entire ecosystem.